As one of methods to observe the systemic hemodynamics or metabolism by using the MRI apparatus, there is proposed a method (moving bed imaging method) that acquires an image while moving a table continuously (see Patent document 1).
With regard to the above moving bed imaging, there are also proposed several means such as using a signal processing to recover image degradation that occurs because the imaging is performed while moving the bed, and limiting a velocity at which the bed is moved to a low speed so that the image degradation may not occur.
The patent document 1 further proposes that in a full-body imaging by MRA (magnetic resonance angiography), a bed moving velocity is made higher for a body trunk whereas it is made slower for lower extremities, so as to perform the imaging with consideration for the imaging area to coincide with a region where a contrast agent is flowing through blood vessels.    Patent document 1: U.S. Pat. No. 6,912,415
Since a living body has a cardiac beat and respiratory movement, those may degrade an image quality when an image of the heart or the liver is acquired. In order to avoid this image degradation due to such movements of the living body, cardiac triggering, pulse triggering, and respiratory gating are widely employed. In addition, as a method to eliminate a body motion artifact that occurs due to the movement of the examined subject while one piece of MR image is generated, there is proposed a method in which a respiratory motion is monitored by using an external breath sensor or a navigator echo, for instance, and measurement is performed only when a position of the examined subject undergoes a predetermined displacement.
Also in the moving bed imaging method as described above, a technique to suppress the body motion is essential to obtain a favorable image. However, if a biological gating signal is applied to the moving bed imaging method, following disadvantages may occur.
That is, according to the biological gating method, an image is not acquired while a gating signal is OFF. However, even while the gating signal is OFF, the bed continues moving. Therefore, positions of the bed are discontinuous in measured data, resulting in that artifact is generated on the image. According to the technical concept to vary the bed moving velocity as disclosed by the patent document 1, the movement of the bed is turned ON and OFF in response to the biological gating signal, in order to avoid the occurrence of such artifact. In this particular case, after gating off, the movement is restarted from the position which is the same as the position before gating off, whereby the imaging positions can be rendered continuous in a series of data acquisition.
However, generally, when the movement of bed is stopped and restarted, a predetermined length of time is needed until the bed movement reaches a constant speed. Therefore, controlling of the speed becomes difficult and this is not practical. In addition, such intermittent driving of the bed may increase a load on the examined subject. Similar problems may occur also in the case where the bed moving velocity is controlled, considering the blood travel time that is different by location, as described in the patent document 1.
An aspect of this disclosure is to provide an approach to reduce a load on the examined subject due to a change of moving velocity of the transport unit, and further provide an image that is effective for diagnosis, responding to variations in conditions while imaging a wide area, in the MRI apparatus employing the moving bed imaging method.
In the conventional art, along with a movement of a bed as a transport unit for a test object, a field of view (FOV) (signal acquisition area) on the examined subject moves in a direction opposite to the direction of the bed movement, both moving at the same velocity. On the other hand, the above-mentioned aspect of this disclosure can involve, for example, controlling the FOV (signal acquisition area) to move at a speed different from the bed movement.
In an exemplary embodiment of this disclosure, a nuclear magnetic resonance imaging method is provided that acquires an image of an imaging region of the examined subject, in a static magnetic field space of a nuclear magnetic resonance imaging apparatus provided with an FOV being desirable, the imaging region being wider than the FOV, while moving the transport unit having the examined subject thereon including, a step (1) that moves the examined subject so that the imaging region of the examined subject passes through the space for imaging, a step (2) that relatively displaces the FOV with respect to the static magnetic field space, in response to information obtained from the examined subject during at least a part of a period when the examined subject is being moved, a step (3) that executes an imaging pulse sequence while the examined subject is being moved, and collects NMR signals from the examined subject, and a step (4) that reconstructs an image of the wide imaging region of the examined subject, by using the NMR signals.
It is to be noted here that the information obtained from the examined subject may be biological information directly acquired from the examined subject such as an electrocardiograph and sphygmograph, for example, further including all the other information that can be obtained regarding the examined subject, such as positional information according to body motion and shifting of the examined subject, and information obtained from the nuclear magnetic resonance signal.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (2) that relatively displaces the FOV, the information obtained from the examined subject is biological information detected from the examined subject, and a moving velocity of the FOV for the examined subject is controlled to be different from each other between in a first period and in a second period while acquiring an image of the imaging region, in response to this biological information.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (2) that relatively displaces the FOV, a direction of the relative displacement is controlled to be different from each other between in the first period and in the second period.
The nuclear magnetic resonance imaging method according to the present invention includes a step (5) that configures settings for a first area and a second area within the imaging region, prior to the step (3) that collects the NMR signals, further includes in the step (2) that relatively displaces the FOV, a step (6) that acquires positional information of the FOV for the examined subject, the information obtained from the examined subject is information indicating reaching that the FOV has reached each of the areas, and a moving velocity of the FOV for the examined subject is controlled to be different from each other between in the first area and in the second area, in response to the information indicating the reaching.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (2) that relatively displaces the FOV, the direction of the relative displacement is controlled to be different from each other in the first area and in the second area.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (3) that collects the NMR signals, imaging of at least a part of the imaging region, for instance, includes a synchronous imaging by using the biological information, the imaging is executed in the first period in response to the biological information, the imaging is suspended in the second period in response to the biological information, the direction of the relative displacement is opposite to the moving direction of the examined subject in the first period, and the direction of the relative displacement is the same as the moving direction of the examined subject in the second period.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (3) that collects the NMR signals, the biological information is acquired from a biological signal, for example, obtained from at least one of the electrocardiograph, the sphygmograph, and a body motion monitor, in the step (1) that moves the examined subject, a moving velocity of the transport unit is determined by a total imaging time including the first period and the second period and a distance the transport unit has traveled for acquiring an image of the imaging region, in the step (2) that relatively displaces the FOV, the moving velocity of the FOV for the examined subject within the first period is determined by the total imaging time when no synchronous imaging is performed, and a distance that the transport unit has traveled for acquiring the image of the imaging region, the imaging is executed from a position of the FOV at the point when the imaging is suspended, and in the second period, the moving velocity of the FOV for the examined subject is set to zero and the position of the FOV for the examined subject is not moved.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (3) that collects the nuclear magnetic resonance signal, imaging of the second area includes, for example, collecting of the NMR signals required for reconstructing a subject image, and collecting of the NMR signals required for detecting a body motion of the examined subject, and in the step (2), the moving velocity of the FOV for the examined subject is controlled to be lower in the second area than in the first area.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (5) that configures settings of each of the areas, an imaging condition in the first area is set to be different from the imaging condition in the second area, for example, and in the step (2) that relatively displaces the FOV, the moving velocity of the FOV for the examined subject is controlled to be different from each other between in the first area and in the second area, in response to each of the imaging conditions.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (5) that configures settings of each of the areas, for example, the imaging conditions are set such that the second area should have a spatial resolution higher than the first area, and in the step (2) that relatively displaces the FOV, the moving velocity of the FOV for the examined subject is controlled to be lower in the second area than in the first area.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (5) that configures settings of each of the areas, for example, the imaging condition for the second area is set in such a manner that at least one of a slice number, a phase encoding number, and a slice encoding number is increased relative to the imaging condition for the first area.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (5) that configures settings of each of the areas, the imaging conditions are set such that the second area should have a higher SN than the first area, and in the step (2) that relatively displaces the FOV, the moving velocity of the FOV for the examined subject is controlled to be lower in the second area than in the first area.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (5) that configures settings of each of the areas, for example, the imaging conditions are set in such a manner that a number of averaging the nuclear magnetic resonance signals is increased in the imaging condition for the second area, relative to the imaging condition for the first area.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (5) that configures settings of each of the areas, for example, the imaging conditions are set in such a manner that a size of the FOV in the first area and a size of the FOV in the second area are different from each other.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (3) that collects the NMR signals includes an imaging using a contrast agent, and in the step (5) that configures settings of each of the areas, the first area and the second area are configured in such a manner that in the second area, the contrast agent moves at a velocity lower than the first area, and in the step (2) that relatively moves the FOV, the moving velocity of the FOV for the examined subject is controlled to be lower in the second area than in the first area.
The nuclear magnetic resonance imaging method includes a step (7) that obtains a mean flow velocity of the contrast agent within the imaging region, prior to the step (5) that configures settings of each of the areas, in the step (1) that moves the examined subject, the moving velocity of the transport unit is assumed as the mean flow velocity, and in the step (2) that relatively displaces the FOV, in order that the moving velocity of the FOV for the examined subject accords with the moving velocity of the contrast agent, a moving direction of the FOV for the examined subject is set to be opposite to the moving direction of the examined subject in the first area, and the moving direction of the FOV for the examined subject is made to be the same as the moving direction of the examined subject in the second area.
In the nuclear magnetic resonance imaging method according to the present invention, for example, in the step (5) that configures settings of each of the areas, the first area and the second area are configured in such a manner that the second area is set in an area where a body axis direction of the examined subject and the moving direction of the examined subject form a larger angle than in the first area, and in the step (2) that relatively displaces the FOV, the moving velocity of the FOV for the examined subject in the moving direction of the examined subject is controlled to be lower in the second area than in the first area.
In the nuclear magnetic resonance imaging method, in the step (2) that relatively displaces the FOV, for example, the FOV is moved along the body axis direction, and the moving velocity of the FOV in the body axis direction is made approximately identical between the first area and the second area.
In the nuclear magnetic resonance imaging method according to the present invention, for example, in the step (3) that collects the nuclear magnetic resonance signal from the examined subject, a high-frequency magnetic field is applied for exciting the FOV, and in the step (2) that relatively moves the FOV, a frequency of the high-frequency magnetic field is controlled so as to control the relative displacement of the FOV.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (1) that moves the examined subject, for example, the moving velocity of the examined subject is kept constant during a period for acquiring an image of the imaging region.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (1) that moves the examined subject, for example, the moving velocity of the examined subject is controlled to be different from each other between in the first period and in the second period.
In the nuclear magnetic resonance imaging method according to the present invention, in the step (4) that reconstructs the image, for example, an image of a part of the imaging region is reconstructed based on the NMR signals that have been obtained so far within at least a part of the period for acquiring an image of the imaging region.
The nuclear magnetic resonance imaging apparatus according to the present invention includes, a transport unit that transports a subject to be examined within a static magnetic field space including a desired FOV, an information obtaining unit that obtains information from the examined subject, a magnetic field application unit that applies a high-frequency magnetic field and a gradient magnetic field to the examined subject, a control means that controls the transport unit and the magnetic field application unit, and a signal processing unit that receives NMR signals generated from the examined subject to construct an image, wherein, in the nuclear magnetic resonance imaging apparatus that acquires the NRM signals while the examined subject is moved by the transport unit, and acquires an image of an imaging region wider than the FOV, the control unit controls the magnetic field application unit in such a manner that the FOV is relatively displaced with respect to the static magnetic field space, in response to the information from the examined subject, during at least a part of the period when the examined subject is moving.
In the nuclear magnetic resonance imaging apparatus according to the present invention, for example, the information obtaining unit that obtains information from the examined subject, obtains biological information from the examined subject, and the control unit controls the magnetic field application unit, in response to the biological information, in such a manner that the moving velocity of the FOV for the examined subject is made different from each other between in a first period and in a second period, while an image of the imaging region is acquired.
In the nuclear magnetic resonance imaging apparatus according to the present invention, for example, the information obtaining unit that obtains information from the examined subject, further obtains positional information of the FOV for the examined subject, and finds information indicating reaching that the FOV has reached the first area and the second area within the imaging region configured in advance, and the control unit controls the magnetic field application unit, in response to the information indicating the reaching, in such a manner that the moving velocity of the FOV for the examined subject is made different from each other between in the first period and in the second period.
In the nuclear magnetic resonance imaging apparatus according to the present invention, for example, the information obtaining unit that obtains the biological information is at least one of an electrocardiograph, a sphygmograph, and a body motion monitor, and the control unit controls the magnetic field application unit in such a manner that the imaging is performed during the first period in response to the biological information, and the imaging is suspended during the second period in response to the biological information.
In the nuclear magnetic resonance imaging apparatus according to the present invention, for example, the unit that obtains the positional information of the FOV includes an encoder provided on the transport unit, and obtains the positional information of the FOV on the examined subject based on information from the encoder and a frequency of the high-frequency magnetic field.
According to the present invention. In an aspect of this disclosure, imaging is performed while the subject to be examined is moved, and the FOV is relatively displaced with respect to the static magnetic field space while the examined subject (i.e., transport unit) is moved. Therefore, a load on the examined subject is reduced, which is caused by fluctuations in the moving velocity of the transport unit, and further, variations in conditions while imaging a wide area are addressed, thereby providing an image effective for diagnosis. As a way of example, in case of a synchronous imaging, even when the imaging is turned ON/OFF according to a gating signal, it is possible to maintain continuity of an area from which a signal is acquired, and an image without artifact can be obtained. With regard to a part of the imaging area, various imaging is available such as obtaining an image of high spatial resolution and an image of multiple slices, according to a request from a user. In the imaging utilizing a contrast agent, it is possible to acquire an image following a moving velocity of the contrast agent.
In another aspect of this disclosure, a moving velocity of the transport unit can be configured with consideration given to a total imaging time. Therefore, even if suspension or restarting of imaging and/or fluctuations in the imaging time occur due to various imaging, an effect caused by heterogeneity of the magnetic field can be suppressed to a negligible degree, and a favorable image can be acquired.